Changes in the North American telephone network have opened the door to new services available to business and residential customers. In particular, Digital Subscriber Line (DSL) offers customers high-speed data communications and basic telephone service using the existing subscriber line. DSL is compatible with existing analog telephone devices for providing plain old telephone service (POTS). DSL supports a high-speed data channel above the 4 KHz band traditionally used for POTS.
However, characteristics in the local loop can degrade the suitability of the channel for data transmission. Analog telephone lines exhibit a wide range of frequency-response characteristics dependant on a number of factors, including distance from the central office, quality of splices, customer premises wiring, line loading, and customer premises equipment.
There is often no way to determine if a telephone line is suitable to provide DSL service without dispatching a telephone company field technician with a bucket truck to test the line at the customer site. The technician analyzes the customer's local loop, including the premises wiring, using a telephone line test set. The test set gets plugged into a telephone jack at the customer premises, and performs a return loss measurement at various frequencies from 0 to 1.1 MHz, normally spaced apart by 4.3 KHz. The test shows the line's frequency response, and can be used for measuring the quality of the line and identifying problems in the telephone company loop plant or customer premises wiring.
The same technique can also be used to diagnose other problems related to frequency response, including slow modem connections, caller ID failure, and sometimes even poor voice quality.
Other test sets may be operated by telephone company technicians located at the customer premises in order to test the quality of the telephone line back to the central office, It is done this way primarily because a clear analog channel is not set up unless a device at the customer premises is off-hook.